1. Field of the Invention
The present invention relates generally to packet network communication and routing and, more particularly, to a method and apparatus for providing Mobile internet protocol (IP) service through a network address translation gateway.
2. Description of the Background Art
Mobile internet protocol (IP) is an Internet Engineering Task Force (IETF) standard communications protocol that is designed to allow mobile device users to move from one network to another while maintaining the same IP address. Mobile IP is described in IETF RFC 3344, published August 2002 and incorporated by reference herein. Mobile IP provides an efficient, scalable mechanism for node mobility within the Internet. Using Mobile IP, nodes may change their point-of-attachment to the Internet without changing their IP address, which allows them to maintain transport and higher-layer connections while moving. As such, Mobile IP is a core technology that the cellular community plans on utilizing to allow mobile devices, such as, cell phones, personal digital assistants and wireless lap tops, to move seamlessly from one IP network to another.
Mobile IP utilizes home agents (HAs) and foreign agents (FAs), or HAs and a mobile node with Collocated Care of Address (CCOA) element. An HA is a router on a mobile node's home network which tunnels datagrams for delivery to the mobile node when it is away from home, and maintains current location information for the mobile node. A FA is a router on a mobile node's visited network which provides routing services to the mobile node while registered. The FA detunnels and delivers datagrams to the mobile node that were tunneled by the mobile node's home agent. For datagrams sent by a mobile node, the FA may serve as a default router for registered mobile nodes. A mobile node's CCOA provides the same function as an FA, but resides within the mobile node. Either FA or mobile node CCOA are used along with a HA.
When a mobile node detects that it has moved to a foreign network, it obtains a care-of address (COA) on the foreign network. The care-of address can be determined from FA or CCOA, depending in on which is being used. The mobile node operating away from home then registers its new care-of address with its HA through exchange of a Registration Request and Registration Reply message with it, possibly via a FA (if used). Datagrams sent to the mobile node's home address are intercepted by its HA, tunneled by the HA to the mobile node's COA, received at the tunnel endpoint (either at a FA or mobile node's CCOA), and finally delivered to the mobile node.
A basic assumption that Mobile IP makes is that FA or CCOA are uniquely identifiable by a globally routable IP address. This assumption breaks down when a mobile device attempts to communicate from behind a network address translation (NAT) gateway. Typically most home routers, wireless or otherwise, employ NATs (gateway router). IP devices which communicate from behind a NAT are reachable only through the NAT's public address or addresses. Mobile IP relies on sending traffic via IP-in-IP tunneling; however, IP-in-IP tunneling does not generally contain enough information to permit unique translation from common public addresses to particular care-of addresses (CoA) of an FA or CCOA which resides behind the NAT. In particular there are no transmission control protocol (TCP) or user datagram protocol (UDP) port numbers available for a NAT to work with. Therefore, IP-in-IP tunnels used by mobile IP generally cannot pass through a NAT.
Therefore, a need exists for a cost-effective method and apparatus for providing mobile IP service through a NAT gateway.